Minimal ed source code

#ifndef _POSIX_SOURCE
# define _POSIX_SOURCE
#endif

#include <stdio.h>
#include <signal.h>

typedef enum { IGN, CTRL_C, CTRL_D } input_t;

input_t last_input = IGN;

void write_ctrl_c(int signum) {
    puts("\n?");
    last_input = CTRL_C;
}

int main(void) {
    struct sigaction handler = { .sa_handler = &write_ctrl_c };

    if (sigaction(SIGINT, &handler, NULL) == -1) {
        fputs("Error registering signal handler\n", stderr);
        return 1;
    }

    for (;;) {
        switch (getchar()) {
        case EOF:
            switch (last_input) {
            case CTRL_D:
                return 0;
            case IGN:
                last_input = CTRL_D;
                break;
            default:
                last_input = IGN;
            }
            break;
        case '\n':
            puts("?");
        default:
            last_input = IGN;
        }
    }
}